In the process of coating a vehicle body, a baking process for heating the coated surface is usually performed after each of an electrodeposition coating, a middle coat coating, a top coat base coating, and a top coat clear coating has been performed. The number of drying furnaces increases when such a large number of baking treatments are performed, and the amount of heat used by the coating process therefore increases. There is a need to reduce the amount of heat for the sake of energy conservation.
The present inventors have previously proposed a vehicle body production method whereby the amount of heat needed to heat a coated surface can be reduced (see Japanese Patent Application Laid-Open Publication No. 2007-229671=Patent Document 1).
FIG. 11 hereof shows the conventional vehicle body production method disclosed in Patent Document 1. As shown in FIG. 11, in step (hereinafter abbreviated as ST) 01, an electrophoretic coating (also called “electrodeposition coating”) is applied to the surface of a vehicle body, and the electrodeposited surface is heated for 20 minutes at 170° C. In ST02, a middle coat coating is applied to the electrodeposition coating. In ST03, the middle coat coated surface is heated for 5 minutes at 70° C. as a first preheating. In ST04, a top coat base coating is applied on the middle coat coated surface. In ST05, the top coat base coated surface is heated for 10 minutes at 80° C. as a second preheating. In ST06, a top coat clear coating is applied on the top coat base coated surface. In ST07, the top coat clear coated surface is heated for 30 minutes at 140° C.
Since heat is applied at a lower temperature and for a shorter time in the first preheating of ST03 and the second preheating of ST05 than in the heating of ST07, the amount of heat needed for heating can be reduced. Such a coating method involves a single heating after three coatings, and is therefore referred to as “three-coat/one-bake” system
Protrusions sometimes appear on the surface of the base electrodeposition coating. The protrusions adversely affect the appearance of the coating, and must therefore be removed to smooth the coating surface. Patent Document 2 discloses a conventional vehicle body production method that includes polishing as an operation for smoothing the electrodeposited surface after the electrodeposition coating is formed.
FIG. 12 hereof shows the conventional vehicle body production method including the polishing operation as disclosed in Japanese Patent Application Laid-Open Publication No. 8-187652 (Patent Document 2). In FIG. 12, the vehicle body is conveyed from left to right. An electrodeposition coating device 300 is composed of an electrodeposition coating bath 302 for applying an electrodeposition coating to a vehicle body 301, a rinsing section 303 for removing excess electrodeposition paint adhering to the vehicle body 301, a vehicle body transfer section 304, a drying furnace 305 for heating the electrodeposited surface of the vehicle body 301, and a polishing section 306 in which polishing of the electrodeposition coating on the outer surface of the vehicle body 301 is performed by a human operator.
In the electrodeposition coating device 300 of Patent Document 2, since the surface of the electrodeposition coating is polished in the polishing section 306, any protrusions that may have formed on the surface of the electrodeposition coating are removed, and a smooth coating surface can be obtained.
However, since the thickness of the electrodeposition coating is very small; i.e., on the order of several tens of micrometers, caution is taken during the polishing so as not to peel off the coating. When polishing is performed carefully, the amount of polishing is deficient to remove an adequate amount of the protrusions, and there is a risk of the protrusions remaining.
The middle coat coating shown in ST02, the top coat base coating shown in ST04, and the top coat clear coating shown in ST06 of FIG. 10 are applied on the electrodeposition coating in which the protrusions remain. When the heating shown in ST07 is then performed, the middle coat coating, top coat base coating, and top coat clear coating conform to the shape of the protrusions as viewed in cross-section, and protrusions therefore appear on the surface of the top coat clear coating. The quality of the coating is reduced when such protrusions occur on the outer surface of the vehicle body.